The photograph shows the jawbone, or mandible, of the phantom midgeMidge bones in lake sediments reveal fish history

(13.05.2009) The mouth parts of the phantom midge are microscopic. But in the hands of scientists from the Department of Zoology at the University of Gothenburg these midge bones become a time machine that can document 200 years of acidification and fish elimination in Swedish lakes.

Acidification of land and water is one of the greatest environmental problems of modern time. Many European lakes still show obvious signs of acidification, and have lead to extensive fish elimination and severely reduced biological diversity.

arlier research has shown a clear connection between fish elimination and larvae of the phantom midge, where a reduction in the fish population can lead to an explosion of phantom midge larvae in acidified lakes.

This invasion of midges forms the basis of a unique research project at the University of Gothenburg. Researchers can now reconstruct the development of fish population in acidified lakes and learn how the population has changed in past centuries by investigating mouth parts of phantom midges that have been preserved in lake sediments.

"What we do can, in fact, be viewed as a journey through time, in which we reconstruct the history of a lake from the early 19th century onwards. We analyse the occurrence of the phantom midge mouth parts and determine which species are present in the sediments.

This allow us to determine whether the number of fish has increased or decreased through history, whether fish have been eliminated completely and disappeared from the lake, and we can also give a rough description of when different fish species have been eliminated during periods of severe acidification", says Fredrik Palm, doctoral student and researcher in the Department of Zoology at the University of Gothenburg.

This method of investigation makes it possible to study the effects of acidification in lakes in which samples have not previously been taken, and where historical information about the fish population is not available.

"Such studies, in turn, make it possible to decide how the biological restoration of an acid lake should be carried out, since it reveals the structure of a fish population that should be the restoration target, in order for the lake to be considered fully restored", says Fredrik Palm.

The historical perspective of the method also makes it possible to survey natural variations in lake ecosystems. In this way, scientists can estimate human impact on lake ecosystems and relate this to climate change, eutrophication and acidification.

Fredrik Palm is carrying out his studies in Västra Götaland and Bohuslän, with a special focus on the Gårdsjön area in Ucklum. This region has been an important centre of Swedish acidification research for decades.